The central feature of Ben Rubin’s multimedia installation is a large, high intensity LED display system furnished by Parallel Development. Mounted behind the glass skin of Adobe’s 18 story headquarters, the Semaphore is visible on the city skyline from considerable distances. The display renders four rotating discs bisected by a dark band, pausing in various positions to communicate textual information in an arbitrary encoding scheme.

San Jose Semaphore was commissioned by Adobe Systems in collaboration with the City of San Jose’s public art program. In support of this civic program, San Jose-based Philips Lumileds donated 26,000 of their Luxeon LEDs to the project, allowing us to develop an exceptionally bright and reliable system.

Rubin’s piece has many other dimensions beyond the display system itself which can be explored in depth at www.sanjosesemaphore.org. Parallel has a long history of working with artist Ben Rubin, and more on his work can be found at www.earstudio.com.

Site

The four discs are visually anchored to the grid created by the curtain wall mullions. The center of each disc is located at the intersection of the framing elements, and the vertical mullions circumscribe the circle. To integrate the semaphore with the existing architecture of the building, Parallel designed a modular system whose panel size and pixel pitch would fit this grid precisely. With a standard, off-the-shelf system this relationship between the image and the architecture would have been impossible.

The frosting of glass cuts the transmission of light and acts a diffuser. To ensure daylight visibility, shop tests were done with glass samples and a variety of Luxeon emitters. The final pixel spacing provides an estimated peak brightness of around 6,500 nits, a level higher than the typical outdoor display standard.

The structural steel support system in the space where the Semaphore displays are mounted presented further complications to the design. Fireproofed steel columns sit inches behind the glass and constrained the possible depth of the display cabinets. To support the 34 foot high curtain wall, steel ‘kicker frames’ brace the wall against wind loads. These structural angles interfered with the positioning of the display cabinets, so Parallel designed a system of reworking these connections.

Working with structural engineer Avery Miller, Parallel designed a simple mounting scheme. The lightweight aluminum display cabinets fit inside the window mullions and bolt directly to the existing curtain wall, eliminating the need for additional support for the display system. Each cabinet makes up one quadrant of a disc and houses 16 display modules which can be individually removed and serviced without removing the cabinets.

Lumiled’s Luxeon LEDs are high intensity devices which need to be mounted to a thermally conductive substrate. Five LEDs are mounted in a line to small aluminum core circuit boards, which in turn bolt onto a CNC milled aluminum plate to form a 10 x 10 pixel display module measuring about 14 inches square. These aluminum plates basically form the structural face of the display cabinet frame; the entire mounting system acts as a massive heatsink to ensure that the LEDs stay cool. This design approach allows the LED strips to be replaced individually should an LED fail.

The 100 monochrome LEDs in each module are driven using constant current LED driver ICs on the driver board bolted to the backside of the aluminum plate. The overall thickness of the display system is just over one inch; most of the 3 inch cabinet depth is taken up by the 40A power supplies which feed each module.

To create the display data, each module uses Parallel’s FPGA based DC6CH display controller. These PCBs decode display data from a single data line feeding the cabinet and refresh the driver ICs. They also provide various diagnostic and control functions to the modules, such as open LED fault detection, global brightness setting and gamma correction.

The brightness of each pixel is individually controlled with 8 bit depth at a refresh rate of 120hz. During the hours of Semaphore operation, the main computer controlling the display alters both the brightness and the gamma correction tables of the display modules to accomodate for ambient light conditions.

System Software

Rubin wanted the discs to rotate as if they were physical objects with inertia; one of his early concepts for the piece was to mount 10′ rotating vanes behind the glass.

Inspired by motion control systems, we developed C++ software that mimics the behavior of a rotational mass accelerated by a motor of limited torque and speed. Each disc has an embedded linux computer system which animates and renders the disc image. Rubin’s central system runs a MAX application which orchestrates the entire system and sends motion trajectory commands to the four disc processors.

The disc processing application was written using Trolltech’s QT embedded library.